Despite concerns, soy probably safe for thyroid

NEW YORK (Reuters Health) - Despite some concerns to the contrary, the soy-based dietary supplement genistein may not harm postmenopausal women's thyroid function, a new study finds.

Genistein is a type of soy isoflavone, a plant chemical that is structurally similar to estrogen and may have certain estrogen-like effects in the human body. In a 2007 clinical trial, Italian researchers found that genistein supplements, along with calcium and vitamin D, appeared to help boost bone mass in postmenopausal women with thinning bones.

In this latest study, the researchers evaluated data from the same clinical trial -- this time looking at whether the genistein supplements had any effects on the women's thyroid function.

The question stems from lab research showing that genistein and other isoflavones may decrease thyroid-hormone production. Thyroid hormones help govern metabolism, and an underactive thyroid gland, called hypothyroidism, can lead to problems like fatigue, weight gain and intolerance to cold.

The earlier research suggested that isoflavones can affect thyroid hormones by interfering with iodine, which is needed for thyroid- hormone production, explained Dr. Francesco Squadrito of the University of Messina, the senior researcher on the study.

However, he told Reuters Health by email, those studies used genistein doses that were 10 to 250 times higher than the doses used in his team's clinical trial -- 54 milligrams (mg) per day.

Squadrito and his colleagues found that among 77 study participants they followed for three years, those who used the genistein supplement during that time showed no overall differences in thyroid function compared with women who were given a placebo.

The findings are published in the Journal of Clinical Endocrinology and Metabolism.

According to Squadrito, it is not surprising that studies would find thyroid effects of very high doses of genistein. However, he said, women are unlikely to consume such levels from soy-protein products, or from soy foods like tofu.

As far as thyroid function is concerned, Squadrito said, "it is possible to conclude that genistein therapy is safe in postmenopausal women -- at least at the dose of 54 mg a day."

However, soy contains several types of isoflavones, and more studies are needed to establish the safety of those compounds, according to Squadrito and his colleagues.